This invention relates to a digital time division, multiplex telecommunications switching system, and more particularly to methods and apparatus providing real time, fault detection and diagnosis of malfunctions.
In general, call processing by a telecommunications switching system is accomplished by connecting a terminal, for transmitting and receiving information, to another terminal by means of a switching network. In a digital time division multiplex telecommunications switching system, the switching network, which includes an information memory or memories, connects one terminal to another terminal "in time" by sequentially storing digital samples of information from first and second terminals in assigned locations of the information memory, swapping the two samples in time, and returning the first and second samples respectively to the second and first terminals.
In providing "in time" call processing between analog information terminals, the information from the terminals must first be sampled (to create successive pulse amplitude modulated, PAM, signals), after which each sample is digitally encoded by analog to digital converters. The successive digital samples (multi-bit binary words which represent data viewable as numbers) from each terminal correspond to a particular channel in the switching system. After sampling and encoding, the digital samples from various terminals are time multiplexed by multiplexers in order to put several channels of information onto a single transmission path or wire with each channel occupying a dedicated time slot and each bit of a sample represented by time-serial "1" or "0" pulses. Whenever a channel is idle, its time slot on the wire will simply be vacant. Time multiplexing of a number of channels serves to minimize the number of physical wires going to the information memories in the switching network. After the swapping has occurred in memory, the information is routed in reverse sequence back through demultiplexers, to digital-to-analog converters and to the connected terminal.
The swapping of samples in the information memories is accomplished under computer control. The computer or control complex (abbreviated as "CPU") also keeps a record of the status of the switching system including the state of the terminals so as to properly process a request for service. In a system using the present invention, the computer may be distributed microprocessors (as described in Pitroda et al United States application Ser. No. 842,091, filed Oct. 17, 1977, which is a continuation-in-part of Ser. No. 734,732, filed Oct. 21, 1976) which control call processing and monitor the status of the system.
In a digital time division, multiplex switching system of a practical size, having as many as three thousand channels, the number and the complexity of the information routing paths to and from the switching network, and the "time" paths within the switching network, produce a complicated maintenance and diagnostic problem. If, for example, there is a failure in the switching network, the wrong channel samples may be swapped. Any such failure appears to be a fault in the system information path including the multiplexers/demultiplexers. The maintenance and diagnostic problem becomes how to detect error in the information path in real time and then to isolate those errors to a minimum number of circuit components in the multiplexers/demultiplexers and the switching network.
Furthermore, telephone switching equipment must have an extremely high degree of reliability. Commonly, only one hour of system down time in twenty years is the reliability goal. It is important, therefore, to detect and identify faults as soon as they occur, so that they may be cured expeditiously or back-up equipment placed into service. Maintenance of the switching system hinges on providing fault detection which is able to rapidly detect real, as opposed to apparent, faulty operation of the system components so that proper remedial action can be taken, i.e., switching in spare circuits or providing fault information so that a craftsperson can replace the faulty components.